1. Field of the Invention
The present invention relates to developing apparatuses in image forming apparatuses.
2. Description of the Related Art
In a conventional developing apparatus based on the two-component (i.e., toner and carrier) developing system, typically two or three screws are disposed in parallel and extending in a horizontal direction. One of the screws is disposed opposite a developing roller and configured to supply a developer containing toner and carrier to the developing roller. The developer is supplied to and collected from the developing roller at the same time by the developer supply screw.
In order to enhance performance for handling color printing and achieve size reduction in a tandem layout, it has been proposed to supply and collect the developer by separate screws, one disposed above the other in a cross section. In such a system, the developer may be circulated in one direction, as discussed in Japanese Laid-Open Patent Publication Nos. 5-333691 (which may be referred to as “Patent Document 1”) and 11-174810 (which may be referred to as “Patent Document 2”). In these examples, a doctor blade is disposed above the developing roller, and the developer is transported from the upper supply screw to the developing roller and then transported to the collecting screw disposed below the developing roller.
JP Patent No. 3950735 (which may be referred to as “Patent Document 3”) discloses a developing apparatus in which the doctor blade is disposed below a pair of developing rollers, one disposed above the other, and that the developer is circulated in one direction. A supply screw is disposed under a collecting screw. The developer is transported up to the top of developing rollers, where the developer is separated at the top of the developing rollers. It is discussed that the level of stress imparted to the developer during its upward transport is reduced.
However, in the above related art, the developer separated from the developing roller may not be collected but again become attached to the developing roller because of the height of the developer accumulated in the downstream end of the collecting screw. Further, the image forming speed may be reduced depending on the type of a printing sheet (such as normal paper or thick paper) used in the image forming apparatus in which the developing apparatus is disposed. The image forming speed may also be reduced in a high-quality mode in which the number of dots formed in a latent image by an electrostatic latent image forming unit may be increased. For these reasons, multiple rotating speeds may be provided. As a result, the height of the accumulated developer may vary depending on the rotating speed of the developer transport screw. For example, when the rotating speed is reduced, the height of the accumulated developer may increase, resulting in a failure to sufficiently collect the developer.